Concrete blocks have been positioned around the Houses of Parliament to protect the building from a possible terrorist attack.

Whitehall officials and the Metropolitan Police insist the action is not in response to a specific threat, but part of a continuing process to increase security in London.

There will also be more visible policing at Heathrow airport.

In the past few weeks, terrorist attacks in Saudi Arabia and Morocco have raised fears that al-Qaeda has regrouped and is again planning to strike western targets.

The BBC's security correspondent Frank Gardner said the primary aim of the increased security "is to stop a suicide truck bomb slamming into the Houses of Parliament", which he said would have been incredibly easy.

The concrete blocks would remain in place for a long time, he added.

The security of Westminster remains under constant review in
the light of the changing assessment of the terrorist threat

Lord Brabazon

"It is not going to stop a determined attacker with really subtle carefully worked out plans, but it is certainly going to deter the casual bomber," he said.

Parliament, however, remains open for visitors during the current recess.

In the last 24 hours Eliza Manningham-Buller the head of MI5, Britain's security forces, has met with George Tenet, the director of the US Central Intelligence Agency (CIA).

They were believed to be discussing the increased threat of terrorism from the al-Qaeda network.

Extra security is also to be set up at Heathrow Airport. Tanks were deployed at the airport for a few days in February.

'Alert not alarm'

Security at Parliament has been tight since the 11 September attacks in the US, with armed police on guard outside the main entrances.

Lord Brabazon of Tara, chairman of the Lords administration, announced the security arrangements on Thursday.